During the manufacture of light sensitive film and paper, three distinct steps are carried out: 1) preparation of the light sensitive emulsion; 2) manufacture of the base or support for the emulsion; and 3) coating of the emulsion on the base. Typically, the emulsion contains silver halide and is prepared by precipitation, washing, and spectral and chemical sensitizations. The emulsion can include addenda such as anti-fogging agents, stabilizers, coating additives and coupler dispersions, all of which are usually added to the emulsion prior to coating. During the process of preparing the emulsion prior to coating, the emulsion is frequently stored in an open vessel. While being stored in an open vessel, it is necessary to continuously agitate the emulsion to prevent the gelatin from forming slugs. It is also necessary to agitate the emulsion to prevent settling of the silver halide particles in the emulsion.
Prior art mixing devices include marine propellers which splash and induce significant entrained air. In addition, while draining a kettle being agitated by a marine propeller, it is necessary to stop the agitation or the air entrainment of the emulsion becomes unacceptable. Finally, propellers do not induce short length scale mixing in the region of microns (micromixing) of the emulsion, rather, they act as a pump, providing long length scale mixing in the region of millimeters (macromixing) without micromixing.
The present invention is a method and apparatus that allows one to agitate an emulsion in an open vessel while inhibiting air entrainment to provide a homogeneous solution to the coating device.